Grouting with high water/solid-ratios. Literature and laboratory study
The grouting methodology in Sweden is developing towards a more extensive use of low water/cement-ratios. The main reason for this development is the better stability of the lower w/c-ratio grouts meaning that the water separation is less than for higher w/c-ratios. However, grouting in low permeable rock or in clay-filled fractures often is performed with high w/c-grouts with better sealing efficiency. Another method that involves high w/c-ratio grouting is “grout thickening” where the grouting starts with a high w/c-ratio and successively lower w/c-ratio is used.
In this study the penetration and the stop mechanisms of suspension grouts have been investigated. The study started with a literature survey, from which hypothesis are formed and tested in the laboratory. The laboratory method used was sand column tests. The column, height 1 m and diameter 0.1 m, was filled with 0.9 m sand which was characterised by hydraulic measurements. The grouting was performed with Myanit, a suspension consisting of crushed dolomite with similar rheological characteristics as cement. The main advantage of using Mynait is that it is an inert material. This implies that it will keep it characteristics throughout the grouting and it will not harden with time. In order to obtain different relationships between grout grain size and theoretical aperture and hence groutability, different ratios between water and the solids were used as well as different sand grain distribution.
Generally, the conclusions are that the penetration increased with higher water/solid-ratios and the penetration stops due to three different mechanisms. In apertures that are too small for the grout to enter, the sealing occurs due to blocking of the entrance. At the limit on what is possible to penetrate, a higher w/s-ratio leads to a further penetration compared to a grout with lower w/s-ratio. The suspension is not moving as a united front, rather it is a more dilute grout in the front, which leads to sealing by single suspensions grains that blocks the pathway. In larger aperture, the grout penetrates more united and the penetration stops due to equilibrium between driving forces and friction forces.
The results implies that the use of “grout thickening” in the field will lead to that the initial higher w/c-ratio grout will penetrate a larger area of the fracture plane and suspension grains will successively plug the constrictions. The thicker grout will then penetrate the larger openings and the combined effect will give decreased permeability compared to only use one w/c-ratio.